1. Field of Invention
The invention relates to a buttonhole sewing machine capable of forming a stitched buttonhole having a pair of left and right zigzag stitch portions placed at both sides of a defined buttonhole and a pair of bar-tack stitch portions disposed at both ends of the zigzag stitch portions on a work cloth.
2. Description of Related Art
A conventional buttonhole sewing machine typically includes a feed bracket that feeds a work cloth, a sewing device that creates stitches on the work cloth, and a sewing controller that controls the feed bracket and the sewing device to form a stitched buttonhole having a pair of left and right zigzag stitch portions placed at both sides of a buttonhole to be formed and a pair of bar-tack stitch portions disposed at both ends of the zigzag stitch portions. In such a buttonhole sewing machine, the sewing controller controls the feed bracket and the sewing device, the work cloth is fed and stitches are made thereon. Thus, a stitched buttonhole 70 (which will be later described in detail), as shown in FIG. 6, can be created.
However, the buttonhole sewing machine does not provide for minute settings regarding buttonhole shapes. For example, the stitch width of the needle to sew the zigzag stitch portions 71,72 (hereinafter referred to as a zigzag width) was fixed to a predetermined value. In short, the buttonhole sewing machine provides for the settings of the size of the stitched buttonhole 70 and the stitch pitch, but the settings of the shape of the stitched buttonhole 70 and the balance between the left and right zigzag stitch portions 71,72 and between the front and rear bar-tack stitch portions 73,74 were not taken into consideration. As a result, problems have occurred. For example, the widths of the left and right zigzag stitch portions 71,72 are usually set so as to be equal to each other. According to the tension of the needle thread, the finish looks as though the widths of the left and right zigzag stitch portions 71,72 are different. However, the buttonhole sewing machine could not correct the balance by means of adjustment. Similarly, it could not correct the balance between the front and rear bar-tack stitch portions 73,74 as if the finish looks as though their widths are different.
When this kind of stitched buttonhole is formed on the work cloth, double sewing, in which the finished stitched buttonhole is sewn again so as to overlap the needle thread on the work cloth, is performed in some cases. However, in double sewing, the needle thread already sewn into the stitches may be broken because the second sewing is made on the same needle points where the first sewing is made. In addition, when double sewing is partially made, a part where double sewing is made and another part where double sewing is not made are different in thickness. Because the conventional sewing machine was provided for a fixed stitched buttonhole pattern only, it could not make the adjustment in proportion to the difference in the stitch thickness.